The present exemplary embodiments relate to liquid crystal displays. It finds particular application in conjunction with polymer stabilized transflective liquid crystal displays, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
Liquid crystal displays (LCD's) can be described as being one of three basic types: transmissive, reflective or transflective. In transmissive construction, all of the light seen by the user is transmitted through the LCD from a backlight. Typically, the LCD pixels are illuminated from behind the display using a cold cathode fluorescent lamp (CCFL) or LED.
Transmissive LCDs offer wide color gamut, high contrast and are typically used in laptop personal computers. Transmissive LCDs offer the best performance under lighting conditions varying from complete darkness to an office environment. In very bright outdoor environments they tend to “wash out” and become unreadable due to the reflection of the sun from the surface of the LCD overwhelming the light being transmitted through the display.
In reflective LCDs, the pixels are illuminated from the front. Reflective LCD pixels reflect incident light originating from the ambient environment or from a frontlight using a mirror layer situated behind the LCD. Reflective LCDs consume very low power (especially without a front light) and are often used in small portable devices such as handheld games, PDA's or other portable instrumentation. Reflective LCDs offer good performance under lighting conditions typical of office environments and brighter. Under dim lighting conditions, reflective LCDs become more difficult to read due to the absence of a strong light source to illuminate the pixels.
Transflective LCDs combine the characteristics of both transmissive and reflective displays. The pixels in a transflective display are partially transmitting and are illuminated by a backlight. The use of a partially reflective mirror also renders the pixels partially reflective, so under ambient illumination they also reflect light from the environment. Transflective LCDs are often used in devices that will be used under a wide variety of lighting conditions (from complete darkness to full sunlight). Under dim lighting conditions transflective LCDs offer visual performance similar to transmissive LCDs, while under bright lighting conditions they offer visual performance similar to reflective LCDs.
A transreflective display can be operated in two modes: reflective mode, in which ambient light is reflected from the display, and transmissive mode, in which light from the backlight passes through the liquid crystal and emitted from the display. It is desired that the operations of the reflective mode and transmissive mode be synchronized, that is, in one voltage condition, both modes are in black state and in another voltage condition, both mode are in bright state. In any other arrangement, the two modes will interfere with each other, resulting in a low contrast ratio.
In the transmissive mode, the light passes through the liquid crystal layer only once (from the back to the front) while in the reflective mode, the light passes through the liquid crystal twice (from the front to the back, where it is reflected by a mirror, and then from the back to the front). This feature of different optical paths of the two modes makes it difficult to synchronize their operation. In a state-of-art transreflective display, the transmissive and reflective modes are operated using two different types of pixels.
FIG. 1 shows a typical transflective LCD device 10. The transflective LCD includes upper and lower substrates 12, 14 with an interposed liquid crystal 16. The upper and lower substrates may alternately be referred to as a color filter substrate and an array substrate. The upper substrate 12 includes a common electrode 18 on its surface adjacent the liquid crystal 16. On the other surface of the upper substrate 12, a retardation film 20 and an upper polarizer 22 are formed.
The lower substrate 14 includes a transparent electrode 24 on its surface adjacent the liquid crystal 16. A passivation layer 26 and a patterned mirror electrode 28 are formed in series on the transparent electrode 24. The patterned mirror 28 and the transparent electrode 24 act together as a pixel electrode. The passivation layer 26 and the patterned mirror electrode 28 have a plurality of transmitting holes 30. On the opposite surface of the lower substrate 14, a lower retardation film 30 and a lower polarizer 32 are formed. A backlight device 34 is arranged below the lower polarizer 32.
In order to form a pixel electrode, a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide) is deposited on the lower substrate 14 and then patterned into the transparent electrode 24.
Next, the passivation layer 26 is formed on the transparent electrode 24. A conductive metallic material having superior reflectivity, such as aluminum (Al) or the like, is deposited on the passivation layer 26 and then patterned to form a reflective patterned mirror electrode 28. In this patterning process, the transmitting holes 30 are formed in portions of the reflective patterned mirror electrode 28.
In this way, the transparent electrode 24 forms transmissive mode pixels at the transmitting holes while the patterned mirror electrodes 28 form reflective mode pixels. As discussed above, this structure results in different cell gaps “d1” and “d2” between the common electrode 18 and the pixel electrode (the reflective electrode 28 and the transparent electrode 24). “d1” denotes the first cell gap between the common electrode 18 and the reflective electrode 28 while “d2” denotes the second cell gap between the common electrode 18 and the transparent electrode 24.
The thickness of the liquid crystal layer in the transmissive mode pixels is twice as thick as the reflective mode pixels, as shown in FIG. 1. Therefore the retardation effect of the liquid crystal on the light is the same for the two modes. That is, since different pixel gaps (the transparent portion and the reflective portion) are formed in the liquid crystal layer, there is no difference in the optical retardation of light passing both through the transparent pixels and through the reflective pixels.
Although the operation of the two modes is synchronized in the display, it is a difficult process to fabricate alternating pixels with two thicknesses. Thus, it would be advantageous to develop a new transflective display in which the thickness of the liquid crystal layer is the same for both transmissive and reflective mode pixels.
Polymer stabilized liquid crystals (PSLC's) are generally prepared by dissolving and photopolymerizing monomers (typically less than 5 wt %) in a liquid crystals matrix to form a polymer network. The polymer used to form such networks is typically a UV curable polymer such as a diacrylate. Due to the advantages realized from PSLC's including improved device stability, it would be useful if they could be used to develop suitable transflective displays.